Removable protective coating for articles of manufacture, such as aeronautical propeller blades



g- 1964 R. F. N. SWINGLER ETAL 3,144,349

REMOVABLE PROTECTIVE COATING FOR ARTICLES OF MANUFACTURE, SUCH ASAERONAUTICAL PROPELLER BLADES Filed March 16. 1960 IN VENTOES. RALPHFMSWWGLER n O a United States Patent O 3,144,349 REMOVABLE PROTEOTEVECOATING FOR ARTH- LES OF MANUFACT, SUCH AS AERONAU- TICAL PROPELLERBLADES Ralph F. N. Swingler, 56 Meadow Way, Letchworth, England, andErnest Douglas Teague, 192 Handside Lane, Welwyn Garden City,Hertfordshire, England Filed Mar. 16, 1960, Ser. No. 15,425 Claimspriority, application Great Britain Apr. 8, 1959 13 Claims. (Cl. 117-6)This invention relates to articles of manufacture having a protectivecoating and to a method of applying the coat- Many metals, which areotherwise suitable for structural purposes, do not have the desiredsurface resistance to erosion, oxidation, and the like. Thus, forexample, aluminum alloy aeronautical propeller blades are strong 3 andof light weight, however, they are not highly resistant to damage fromthe abrasive action of particles of Water, sand, stones and othermaterial prevalent on runways. Flame-sprayed coatings, such as disclosedin United States Patent No. 2,707,691, granted to William M. Wheildon,In, May 3, 1955, provide an excellent erosion-resistant surface;however, such coatings are not suitable for direct application toaeronautical propellers because in order to attain the high degree ofadhesion necessary the surface must be modified, for example mymechanical roughing which is undesirable; because the propeller bladesmust frequently be inspected for incipient failure and once a coatinghas been applied, according to the foregoing method of application, itcannot be removed without damage to the propeller; and finally, becausethe coating is not entirely impervious and hence is a starting point forcorrosion.

Objects of this invention are to provide an article of manufacturehaving a coating applied thereto which embodies the advantages ofresistance to abrasion and oxidation of coatings of the kind disclosedin the Wheildon patent, and the further advantages that it is bonded tothe surface without modification of the latter; that it iscorrosion-resistant; and that it is removable without damage to thepropeller. Other objects are to provide a method of attaching aflame-sprayed coating, such as described in the aforesaid Wheildonpatent, without having to modify the surface to which it is to beapplied preparatory to its application except for cleaning; to provide amethod of attaching the coating in such fashion that it will adheretenaciously for the purpose of use, but can easily be removed whendesired; and to provide a method of making the coatingcorrosion-resistant.

In accordance with the foregoing, the article which may be a propellercomprises a rigid structure having a multi-layer coating applied theretoincluding an abrasive resistant layer and a substantiallycorrosion-resistant bonding layer adhesively uniting theabrasive-resistant layer to the structure, but which may be degraded bymeans of an agent which is not harmful to the structure to permitremoving the entire coating. The coating also includes an intermediatemetal layer between the abrasiveresistant layer and the bonding layerand may optionally have an exterior layer applied to theabrasive-resistant layer to reduce porosity of the latter.

The method comprises cleaning the surface of the structure which is tobe coated, applying a layer of synthetic liquid resin to the surface,allowing the resin layer to dry, and then curing it at a temperaturebelow that which would adversely affect the physical properties of thestructure. The method contemplates including with the resin an admixtureof a powdered metal filler and, when the latter is employed, the surfaceof the layer, after curing, is roughened to expose the particles.Optionally,

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the filler may be omitted and a powdered metal deposited on the resinlayer before curing, so that it remains exposed on the surface. Aflame-sprayed intermediate layer of metal is then applied to the roughsurface of the resin layer and finally a flame-sprayed external layer ofimpact resistant material is applied to the intermediate layer.

The invention will now be described in greater detail with reference tothe accompanying drawings wherein:

FIG. 1 discloses a section through a metal structure having amulti-layer coating applied to one side and in which the bonding layerincludes a filler, it being understood that the coating may be appliedto both sides of the structure;

FIG. 2 is a corresponding section in which the filler has been omittedfrom the bonding layer and powdered metal has been applied to thesurface of the bonding layer prior to application of the intermediatelayer;

FIG. 3 is a modification on which a sealing layer is applied to theexternal surface of the multi-layer coating; and

FIG. 4 is a side elevation of an aeronautical propeller with one of theblades shown in cross section.

Referring to the drawings, FIG. 1, there is shown a base 10 which hasapplied to it a coating 12 of multilayer construction. While shownapplied to one surface it is to be understood that the coating may beapplied to as little or as much of the external surface of the base asis desired. The base itself is rigid and suitable for manufacture ofstructural members or articles. The coating 12 includes an inner bondinglayer 14 next to the base, an intermediate layer 16 and an externalabrasive-resistant layer 18.

The bonding layer 14 is a corrosion-resistant layer of adhesivesynthetic resin in a cured condition containing metal powder particles,some of which are exposed at the surface of the resin layer.Alternatively, the metal powder particles may be applied only to thesurface of the resin so that when the latter is cured they project partway out of the surface. A resin is chosen for the bonding layer whichhas a high adhesive strength and preferably is an epoxy resin.

The intermediate layer 16 is a flame-sprayed metal applied to theroughened surface of the bonding layer by well-known methods, forexample, with a Schoop gun.

The external abrasive-resistant layer 18 may be of hard metals,refractory metal carbides or fused metal oxides, or combinations ofmetal oxides including silicon dioxide, flame-sprayed on theintermediate layer 16, according to the method disclosed in the Wheildonpatent, 2,707,- 691. According to the aforesaid patent a rod consistingof a stable metal oxide is fed into a flame hot enough to melt it,whereupon it is instantly atomized to form discrete molten particles andwhile still molten the particles are projected with a blast of gas ontothe surface to be coated. Oxides and carbides applied in this fashionare extremely stable and highly resistant to impact abrasion.Optionally, a sealing layer 20 (FIG. 3) may be applied to the layer 18to reduce its porosity. An epoxy resin is preferred.

As thus made up, the multi-layer coating 12 embodies highly desirablecharacteristics for protecting structural members and articles ingeneral and in particular aeronautical propellers for it provides highimpact resistance to surface damage, is corrosion-resistant, may beapplied without modifying the surface of the propeller preparatory toits application, and may be removed by degrading the bonding layerwithout damage to the propeller.

In its more specific application to aeronautical propellers and inaccordance with the method herein disclosed, the aluminum alloypropeller is thoroughly cleaned after its surface has been anodized,whereupon a layer of synthetic resin is applied to the clean surface.The synthetic resin is preferably an epoxy resin, in particular, aliquid resin, such as that known under the trade name Araldite, however,it is to be understood that any resin having the same or equivalentproperties of corrosionresistance and adhesive strength may be employed.According to one method of preparing the bonding layer, a filler ofpowdered alumina or aluminum alloy is mixed therewith in approximatelyequal volumes. The admixture of resin and filler is then sprayed uponthe surface to a thickness between 0.002" and 0.008" and preferably0.005".

The layer of synthetic resin is allowed to dry and is then cured byheating the propeller at a temperature within the range of 80 C. to 100C. By choosing a resin having a low curing temperature, curing may beeffected without adversely affecting the physical properties of thepropeller. After curing, the surface of the synthetic resin layer ismechanically roughed with shot or abrasive to expose particles of thealuminum filler.

Alternatively, the filler may be omitted and a layer of powderedaluminum or aluminum alloy, preferably dendritic is applied to thesurface of the resin prior to curing, leaving the particles exposed to adepth of /3 of their diameter. As thus prepared the surface need not beroughened by mechanical means. The exposed particles, applied in eitherWay, provide a keying surface which enhances attachment of theintermediate layer thereto.

The intermediate layer 16 is next applied by flamespraying and consistsof an aluminum or aluminum alloy deposited by means of a Schoop gun, orthe like, on the rough surface of the resin layer. The flame-spraying ofmetals is well-known and the method using metal wire, referred to in theWheildon patent, is preferred.

The impact resistance layer 18, preferably is applied according to themethod described in the Wheildon patent and may be comprised of fusedmetal oxide or combinations of metal oxides, including silicon dioxide.The layer is conveniently fused aluminum oxide and is between 0.010 and0.025" thick and preferably 0.020" thick.

Optionally, a sealing layer of epoxy resin 20 may be applied to theabrasive-resistant layer and cured. This layer is between 0.002 and0.005" thick and preferably 0.003". It has the effect of sealing theotherwise somewhat porous surface of the impact resistant layer and isnot always necessary.

The epoxy resin employed for the bonding and sealing layers, referred toherein as Araldite, is manufactured by the Ciba Co., Inc., PlasticsDivision, Kimberton, Pennsylvania. Araldite is a solvent solution of anepoxy resin of the 4,4 bisphenylol glycidyl ether type having coldcuring amine hardeners which can be accelerated by the application ofheat. Epoxy resins are remarkable for the unusual combination ofproperties they possess and, in particular, they combine unsurpassedadhesive strength with hardness, impact resistance and flexibility. Atthe same time, epoxy resins may be degraded when exposed to acetone,ethyl acetate, sulphuric acid, and chlorinated solvents. One suchsolvent is sold under the proprietary name of Ardrox 20, manufactured byBrent Chemical Products Limited, Commerce Road, Brentford, Middlesex,England. Ardrox 20 is a mixture of methylene cellulose, ammonia, wax andmethylene dichloride and is entirely soluble in water.

The multi-layer coating, as thus applied, has the effect of reducing theabrasion of the blades, adheres to the blades, even at the highrotational speed encountered, resists corrosion, and yet may be renderedremovable for inspection of the blades by treatment with a resindegrading agent such as referred to above.

Sometimes it may be desirable to etch the propeller blades with an etchprimer, such as that sold under the name of Celletch, manufactured byCanadian Pittsburgh Industries Limited, Paints Division, Montreal,Canada.

4. Celletch is a mixture of phenolic resin, polyvinyl butyrol, talc andzinc chromates with phosphoric acid and alcohol solvents.

It is to be understood that the term aluminum alloy blade is intended toinclude any blade of aluminum alloy. whether it has an oxide surface ornot. In most cases, the aluminum alloy blades are anodized beforeapplying the coating described herein.

It should be understood that the present disclosure is for the purposeof illustration only and that this invention includes all modificationsand equivalents which fall within the scope of the appended claims.

We claim:

1. The method of coating a rigid structural member which comprises,depositing a layer of corrosion-resistant adhesive resin on the surfaceof the member in an uncured liquid condition, curing the layer, applyinga flamesprayed layer of metal to the cured layer and then applying animpact resistant flame-sprayed layer of a member selected from the groupconsisting of stable metal oxides and refractory metal carbides.

2. A method according to claim 1 wherein the resin layer has admixedtherewith a metal powder and the surface of the resin layer, aftercuring, is treated to expose the metal powder particles.

3. A method according to claim 1, wherein a layer of metal powder isapplied to the surface of the resin layer prior to curing, so that themajor portions of the powder particles project from the surface.

4. A method according to claim 1, wherein a sealing layer of syntheticresin is applied to the external surface of the impact resistant layer.

5. The method of coating an aluminum alloy propeller which comprisescleaning the blades, applying an uncured layer of epoxy resin thereto,curing the resin layer at a temperature below that which deleteriouslyaffects the propeller, treating the resin layer to produce a roughenedsurface, applying a flame-sprayed layer of aluminum to the roughenedsurface of the resin layer, and thereafter applying an impact resistantflame-sprayed layer of a member selected from the group consisting ofstable metal oxides and refractory metal carbides.

6. A method according to claim 5, wherein the propeller blades areinitially anodized.

7. A method according to claim 5, wherein the propeller blades areinitially etched.

8. A method according to claim 5, wherein the resin layer has admixedtherewith a metal powder and the surface of the resin layer, aftercuring, is treated to expose the metal powder particles.

9. A method according to claim 5, wherein a layer of metal powder isapplied to the surface of the resin layer prior to curing, so that themajor portions of the powder particles project from the surface.

10. A method of operating an airplane propeller in an unusually abrasiveenvironment wherein the propeller is protected by an abrasion resistantouter layer which must be removed from time to time for inspection ofthe propeller comprising depositing a layer of corrosion resistantadhesive resin on the surface of the member in an uncured condition,curing the resin layer in situ, applying a flame-sprayed layer of metalto the cured layer, then applying an impact resistant flamesprayed layerof a member selected from the group consisting of stable metal oxidesand refractory metal carbides, driving the propeller until inspection isrequired, and then degrading the cured resin and removing theflame-sprayed coatings to bare the propeller surface for inspection.

11. A method according to claim 10, wherein the resin layer has admixedtherewith a metal powder and the surface of the resin layer, aftercuring, is treated to expose the metal powder particles.

12. A method according to claim 10, wherein a layer of metal powder isapplied to the surface of the resin layer prior to curing, so that themajor portions of the powder particles project from the surface.

13. As an article of manufacture a rigid metal aeronautical propellerhaving a protective multi-layer coating applied thereto, said coatingincluding a rigid external layer of material that is highly resistant toimpact abrasion, said layer comprised of a member selected from thegroup consisting of stable metal oxides and refractory metal carbidesand a bonding layer of from about .002" to .008" thickness, the adhesivestrength between the successive layers and the propeller beingsufficiently great to resist the centrifugal forces encountered duringrotation of the propeller and said bonding layer being degradable bymeans of an agent Which is not harmful to the propeller.

References Cited in the file of this patent UNITED STATES PATENTS2,398,108 Mott Apr. 9, 1946 2,662,831 Culverhouse Dec. 15, 19532,707,691 Wheildon May 3, 1955 2,767,105 Fletcher Oct. 16, 19562,776,225 Fletcher Jan. 1, 1957 2,852,421 Bergstedt Sept. 16, 19582,913,813 Homer et al. Nov. 24, 1959 2,934,456 Schutt Apr. 26, 19603,006,782 Wheildon Oct. 31, 1961 OTHER REFERENCES Polymer Progress, May1955, Shell Chemical Corporation, page 4.

1. THE METHOD OF COATING A RIGID STRUCTURAL MEMBER WHICH COMPRISES,DEPOSITING A LAYER OF CORROSION-RESISTANT ADHESIVE RESIN ON THE SURFACEOF THE MEMBER IN AN UNCURED LIQUID CONDITION, COURING THE LAYER,APPLYING A FLAMESPRAYED LAYER OF METAL TO THE CURED LAYER AND THENAPPLYING AN IMPACT RESISTANT FLAME-SPRAYED LAYER OF A MEMBER SELECTEDFROM THE GROUP CONSISTING OF STABLE METAL OXIDES AND REFRACTORY METALCARBIDES.